This intersection comprises artificial photosynthesis ... Engineered and Artificial Photosynthesis: Human ..... ummary%20Report.pdf (accessed January 2008).
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10. H.S. Kheshgi, R.C. Prince, Energy 30, 1865 (2005). 11. L.R. Lynd, C.E. Wyman, T.U. Gerngross, Biotechnol. Prog. 15 (5), 777 (1999). 12. U.S. Department of Energy, Office of Science, Breaking the Biological Barriers to Cellulosic Ethanol, a Joint Research Agenda, a Research Roadmap Resulting from Biomass-to-Biofuels Workshop, Rockville, December 2005 (DOE/SC 0095, June 2006). 13. C.E. Wyman, Annu. Rev. Energy Environ. 24, 189 (1999). 14. L.R. Lynd, R.T. Elander, C.E. Wyman, Appl. Biochem. Biotechnol. 57–58, 741 (1996). 15. C.E. Wyman, Trends Biotechnol. 25 (4), 153 (2007). 16. Chem. Eng. Prog. 100 (12), 15 (2004). 17. Chem. Eng. News 83 (17), 10 (April 25, 2005).
4. N. Mosier, C.E. Wyman, B. Dale, R. Elander, Y.Y. Lee, M. Holtzapple, M. Ladisch, Bioresour. Technol. 96 (6), 673 (2005). 5. C.E. Wyman, Applied Energy Technology Series, 424 (Taylor and Francis, Washington, DC, 1996). 6. U.S. Department of Energy, Energy Information Administration, Annual Energy Review 2005 (DOE Report DOE/EIA-0384, 2005). 7. R. Perlack, L. Wright, A. Turhollow, R. Graham, B. Stokes, D. Erbach, Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply (Oak Ridge National Laboratory, Oak Ridge, TN, 2005). 8. A.E. Farrell, R.J. Plevin, B.T. Turner, A.D. Jones, M. O’Hare, D.M. Kammen, Science 311, 506 (2006). 9. C.E. Wyman, Appl. Biochem. Biotechnol. 45–6, 897 (1994).
Engineered and Artificial Photosynthesis: Human Ingenuity Enters the Game Devens Gust (Arizona State University, USA), David Kramer (Washington State University, USA), Ana Moore (Arizona State University, USA), Thomas A. Moore (Arizona State University, USA), and Wim Vermaas (Arizona State University, USA) All oxygen-dependent life depends on photosynthesis. In addition to breathing the oxygen produced by photosynthesis, humans have been harnessing energy from photosynthesis for millennia. Since the beginning of human societal structures, human needs have driven the evolution of agricultural production, and they continue to do so. Recently, it has been suggested that agriculture can contribute substantially to human technological (nonnutritional) energy needs. This possibility raises concern because the projections of human energy needs argue convincingly that without large increases in energy conversion efficiency (ECE), land-grown biofuel production and food production will compete for land, a largely untenable compromise given the current nutritional status of the world’s underdeveloped societies. In addition to using the fuel provided by nature’s photosynthetic process, humans have devised direct routes for harnessing solar energy including, for example, photovoltaic (PV) cells. These cells produce energy in the form of electromotive force (emf, electricity), which, although ideal for many applications, is not easily stored and used for fuel (e.g., in transportation). We posit that transformational progress toward meeting the goals of supplanting fossil fuels, providing energy security, and mitigating climate change can be made at the intersection of technology and biology. This intersection comprises artificial photosynthesis, other bio-inspired energy conversion processes, and the design of organisms that specialize in efficient biofuel production from solar energy. As outlined here, artificial constructs can contribute directly to solar energy conversion, can be incorporated into hybrid systems, and can inform the design of new photosynthetic organisms.
elementary photophysical processes; the essential ones are shown in Figure 1. The absorption of light (red and green arrows) promotes an electron to a higher energy level, which leads to an excited state in which an electron is repositioned in spatial and energy coordinates and a positive charge (hole) is left behind. This is the transformation of solar to chemical energy; the electron is chemically reducing (low electrochemical potential), and the hole is chemically oxidizing (high electrochemical potential). In molecular systems, the further stabilization necessary to prevent wasteful relaxation back to the ground state involves moving the electron and hole farther apart; there is a concomitant loss of energy (illustrated by the dash-dotted arrows in Figure 1) necessary to drive this charge separation process. In typical PV cells, the hole and electron are separated and thereby stabilized by an internal electric field at the junction of the n- and p-type semiconductor materials. The energy associated with separating the charges (dash-dotted arrows in Figure 1) reduces the electrical energy available in the external circuit. Charge separation sets the stage for describing three efficiency-defining processes: a high fraction of the photons absorbed must yield charge separation (i.e., the quantum yield of charge separation must be high); the energy of the charge-separated state must be high; and recombination of the electron and hole, producing heat, must be much slower than chemical reactions making productive use of the oxidation and reduction potential (or slower than the conduction of charge in a PV device). ECE is defined as the usable electrical or harvestable chemical energy output divided by the total solar energy incident on the organism or device. In terms of meeting human energy needs, which are usually expressed on an annual basis, it is convenient to calculate ECE using insolation (incident solar energy) per year summed over diurnal and seasonal cycles. ECE is a fundamental parameter that determines the area required to provide a specified amount of energy for human use. Some examples of the ECEs of
What Do We Mean by Efficient, and Why Isn’t Natural Photosynthesis More Efficient?
The initial energy-conserving steps in the conversion of solar energy to either electricity or biomass can be described by
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biofuel production by photosynthetic organisms and electrical survival of the organism (Figure 2, arrow labeled “Growth and work by a typical PV cell are listed in Table I. maintenance”). These processes require copious amounts of Clearly, not all of the energy of incident sunlight can be energy and resources, but do not directly produce harvestable, converted into useful work. In addition to the efficiencyhigh-energy-content biomass for human use. limiting processes mentioned above, other factors come into play. The ECE is limited first by the fraction of sunlight absorbed by the organism or device: photons having less energy than required for the lowest-energy absorption transition of the material will not be absorbed. This is illustrated by the pink arrow in Figure 1. The ECE is also limited because E Lowest excited state both photosynthetic organisms and simple PV devices such as n silicon solar cells function as single-threshold systems. In e other words, the energy of absorbed photons that is above the r Bandgap g lowest excited singlet state of chlorophyll (or the conductionWork out or y band energy of a solar cell) is lost as heat during the electronic threshold relaxation processes that populate the lowest excited state (see arrow labeled “Internal conversion” in Figure 2 and the dashed Ground state arrow in Figure 1). In photosynthesis, this leads to a loss of about one-fourth of the absorbed energy. A threshold or bandFigure 1. Simplified diagram of a single-threshold solar gap of about 1.3 eV is optimal. The increase in chemical potenenergy conversion device. Photons with energy higher than tial of the charge-separated state with increasing bandgap the bandgap are thermalized to the energy of the bandgap comes at the expense of the fraction of photons (in the pink (dashed arrow). Photons with energy lower than the bandgap arrow category in Figure 1) absorbed from the solar spectrum. are not absorbed (pink arrow). Energy loss to separate The bandgap in water-oxidizing photosynthesis is about 1.8 eV, charge and to prevent charge recombination and drive the well on the high-energy side of optimal. Finally, as mentioned system forward to do chemical or electrical work is shown above, conversion devices—natural and artificial—sacrifice by dash-dotted arrows. some of the potentially available energy in order to slow charge recombination reactions and drive desired chemical or elecTable I: Annual Biofuel Production and Energy Conversion Efficiency by Photosynthetic trical processes forward. As a Organisms and Electrical Energy Production by a Photovoltaic Cell. result of these factors, illustrated by the dash-dotted arrows in Oil Producer Fuel Production [kg/(ha Energetic Equivalent ECE (%) year)] [kWh/(ha year)] Figure 1, a significant fraction of photon energy must be lost as Oil palm 3,600–4,000 33,900–37,700 0.16–0.18 heat in any conversion device. Jatropha 2,100–2,800 19,800–26,400 0.09–0.13 The best single-threshold photovoltaic devices have ECEs that Tung oil tree 1,800–2,700 17,000–25,500 0.08–0.12 approach the Shockley–Quiesser (China) limit of about 30%, which was Sugarcane 2,450 16,000 0.08 calculated taking all of the above Castor oil plant 1,200–2,000 11,300–18,900 0.05–0.09 considerations into account. In contrast, although the initial Cassava 1,020 6,600 0.03 steps of plant and bacterial Microalgae 91,000 956,000 4.6 photosynthesis often have very 6 Si-based PV 3 × 10 14.3 high quantum yields, the ECE of cell natural photosynthesis is relatively low (maximally about Source: References 14,15. 6% but usually observed to be Note: ECE (energy conversion efficiency) is calculated by dividing the energetic equivalent by the energy content of the total solar spectrum averaged over 1 year incident on 1 hectare (ha) for a sunny climate at moderate latitude [21 million
